1. Preparation: Rinse the fennel bulbs, trim, and cut horizontally into 1/4-inch (6-mm) thick slices. Thinly slice the onion, dice the apple. Peel the potatoes, cut into 1/4-inch (6-mm) thick slices. It is important to partially cook each of the four main ingredients, fennel, onion, apple, and potatoes, prior to layering in the pan to maintain their flavors and reduce baking time.

Fennel, Onion, and Apple: In a non-stick pan over medium heat, warm the oil and 1 tablespoon (14 g) butter until the butter is melted. Cook the sliced fennel for 3 to 4 minutes, add the sliced onion and cook for 2 minutes then add the diced apple and cook for 2 minutes. The fennel, onion, and apple should be barely fork tender and lightly caramelized but not overly browned or mushy. Remove from the heat, taste and adjust salt as needed.

Potatoes: Cook the potatoes by boiling in lightly salted water for 3 minutes, drain, and reserve.

2. Preheat the oven to 400º F/200º C/Gas Mark 6. Assembling the Gratin: Select a glass or ceramic baking pan, about 8-inches (20-cm) in diameter. Use the remaining 1 tablespoon (14 g) butter to evenly coat the inside of the pan.

Bottom Layer: Arrange the potato in overlapping slices, like a deck of cards spread out. Sprinkle half of the grated Parmigiano-Reggiano then half of the chopped Provolone over the potatoes. Lightly salt to taste.

Top Layer: Arrange an even layer of cooked fennel, onion, and apple over the sliced potatoes and cheese. Drizzle the water over to maintain moisture and prevent the bottom and edges from burning during baking. Sprinkle the remaining half of both cheeses on top.

3. Baking the Gratin: Combine the breadcrumbs, pine nuts, and salt. To keep the topping moist during baking, drizzle in the olive oil and toss. Sprinkle the breadcrumb mixture in an even layer on top of the layered vegetables. At this point the gratin will be about 1-inch (2.5-cm) high.

Place the pan in center of the preheated oven, leave uncovered, and bake for 25 to 30 minutes or until the topping becomes golden brown and the gratin is fragrant. Remove from the oven then enjoy!